首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   4858篇
  免费   499篇
  国内免费   429篇
电工技术   88篇
综合类   772篇
化学工业   99篇
金属工艺   19篇
机械仪表   47篇
建筑科学   2166篇
矿业工程   956篇
能源动力   97篇
轻工业   58篇
水利工程   724篇
石油天然气   94篇
武器工业   16篇
无线电   102篇
一般工业技术   71篇
冶金工业   59篇
原子能技术   6篇
自动化技术   412篇
  2024年   13篇
  2023年   53篇
  2022年   125篇
  2021年   155篇
  2020年   169篇
  2019年   113篇
  2018年   94篇
  2017年   149篇
  2016年   192篇
  2015年   220篇
  2014年   321篇
  2013年   308篇
  2012年   353篇
  2011年   381篇
  2010年   366篇
  2009年   321篇
  2008年   345篇
  2007年   424篇
  2006年   373篇
  2005年   275篇
  2004年   227篇
  2003年   175篇
  2002年   124篇
  2001年   99篇
  2000年   85篇
  1999年   80篇
  1998年   57篇
  1997年   39篇
  1996年   30篇
  1995年   37篇
  1994年   27篇
  1993年   11篇
  1992年   14篇
  1991年   9篇
  1990年   5篇
  1989年   4篇
  1987年   1篇
  1986年   4篇
  1985年   2篇
  1984年   4篇
  1983年   2篇
排序方式: 共有5786条查询结果,搜索用时 15 毫秒
51.
针对目前村镇建设用地集约节约利用效果评价方法空缺及信息化低等问题,依据村镇建设用地节约集约利用效果评价模型,在地理信息系统组件技术和数据库技术支持下,采用可视化编程语言MicrosoftC#.net和GIS控件技术开发了村镇建设用地节约集约利用效果评价系统。以太仓市陆渡镇为开发案例,探讨了系统分析、设计和功能实现等主要技术问题。并利用该系统对太仓市陆渡镇村镇建设用地集约节约利用效果评价进行了评价,评价结果表明,该系统能够满足当前村镇建设用地集约节约利用效果评价工作的需要,为快速准确地实施节地效果评价提供技术支持。  相似文献   
52.
The radiation budget at the earth surface is an essential climate variable for climate monitoring and analysis as well as for verification of climate model output and reanalysis data. Accurate solar surface irradiance data is a prerequisite for an accurate estimation of the radiation budget and for an efficient planning and operation of solar energy systems.This paper describes a new approach for the retrieval of the solar surface irradiance from satellite data. The method is based on radiative transfer modelling and enables the use of extended information about the atmospheric state. Accurate analysis of the interaction between the atmosphere, surface albedo, transmission and the top of atmosphere albedo has been the basis for the new method, characterised by a combination of parameterisations and “eigenvector” look-up tables. The method is characterised by a high computing performance combined with a high accuracy. The performed validation shows that the mean absolute deviation is of the same magnitude as the confidence level of the BSRN (Baseline Surface Radiation Measurement) ground based measurements and significant lower as the CM-SAF (Climate Monitoring Satellite Application Facility) target accuracy of 10 W/m2. The mean absolute difference between monthly means of ground measurements and satellite based solar surface irradiance is 5 W/m2 with a mean bias deviation of − 1 W/m2 and a RMSD (Root Mean Square Deviation) of 5.4 W/m2 for the investigated European sites. The results for the investigated African sites obtained by comparing instantaneous values are also encouraging. The mean absolute difference is with 2.8% even lower as for the European sites being 3.9%, but the mean bias deviation is with − 1.1% slightly higher as for the European sites, being 0.8%. Validation results over the ocean in the Mediterranean Sea using shipboard data complete the validation. The mean bias is − 3.6 W/m2 and 2.3% respectively. The slightly higher mean bias deviation over ocean is at least partly resulting from inherent differences due to the movement of the ship (shadowing, allocation of satellite pixel). The validation results demonstrate that the high accuracy of the surface solar irradiance is given in different climate regions. The discussed method has also the potential to improve the treatment of radiation processes in climate and Numerical Weather Prediction (NWP) models, because of the high accuracy combined with a high computing speed.  相似文献   
53.
以江苏省宜兴地区为试验区,首先进行了CBERS-02星CCD数据的图像预处理,然后利用ERDASI-MAGINE和Definiens7.0等软件,采用非监督分类、监督分类、混合监督分类、面向对象四种分类方法进行了土地利用分类,并将影像分类结果进行了精度评价分析。结果表明:在一级地类的提取中,面向对象的最邻近分类器的分类精度最低,混合监督分类的精度最高;在二级地类的提取中,面向对象的特征阈值法得到的分类结果较好,能够轻松提取传统分类方法难于提取的地类。  相似文献   
54.
Full instrumental rationality and perfect institutions are two cornerstoneassumptions underlying neoclassical models. However, in the real world, thesetwo assumptions never hold, especially not in developing countries. In thispaper, we develop a game theoretical model to investigate if relaxations inthe full instrumental rationality and perfect institutions premise can explainthe conflicts that have been occurring between the various principals in theNarok district in Kenya with regard to land tenure and use.  相似文献   
55.
在西部干旱地区工程项目建设过程中,大规模的开挖、修路等施工会产生大量的弃渣,不仅加剧水土流失,而且造成一系列的生态问题。通过对弃渣状况进行研究,阐述生态修复的工程措施,为干旱地区工程建设项目的弃渣治理提供一定的借鉴意义。  相似文献   
56.
Suspended sediment and organic contaminants were measured during a period of 2 years in the San Lorenzo River, central California, which discharges into the Pacific Ocean within the Monterey Bay National Marine Sanctuary, in an effort to quantify the potential environmental effects of storm events from a steep, mountainous coastal urban watershed. Most suspended sediment transport occurred during flooding caused by winter storms; 56% of the total sediment load for the 2‐year study was transported by the river during one storm event in January 2010. Concentrations of polycyclic aromatic hydrocarbons can exceed regulatory criteria during high‐flow events in the San Lorenzo River, and total annual polycyclic aromatic hydrocarbon load was on the order of 10 kg in water year 2010. These results highlight the importance of episodic sediment and contaminant transport in steep, mountainous coastal watersheds and emphasize the importance of understanding physical processes and quantifying chemical constituents in discharge from coastal watersheds on event‐scale terms. Published 2012. This article is a U.S. Government work and is in the public domain in the USA.  相似文献   
57.
Through investigating the reactions of commercial farmers to land and water reforms in the Trichardtsdal-Ofcolaco area, Limpopo Province, Olifants Basin, South Africa, from 1997 to 2006, it is shown that water claims are key to land redistribution processes, and that commercial farmers make strategic use of arguments for nature conservation and ecological stewardship to defend their claims to water. Given these observations, caution is warranted with respect to the implementation of land and water reforms as separate policy packages; it may be more effective to design water and conservation policies as an integral part of land reform programmes.  相似文献   
58.
为更好地对河流水电开发后的鱼类进行保护,探讨中型山区河流水电站下游的鱼类生态需水量计算.参照大型河流鱼类水力生境参数标准,根据中型山区河流的特点,得到中型山区河流鱼类水力生境参数的参考标准.选取有代表性的四川中型山区河流杂谷脑河进行鱼类生境原型观测,原型观测结果证实了该参考标准能够满足杂谷脑河鱼类对生境的实际需求.基于所得到的参考标准,应用生态水力学法对杂谷脑河红叶二级、理县以及甘堡3座电站减水河段的鱼类生态需水量进行计算分析,得到3座电站的生态需水量推荐值.通过分析可知,所得到的生态需水量推荐值占年均天然流量以及设计引用流量的比例都不高,在3座电站的运行实践中,具有可操作性.  相似文献   
59.
从陆战场空域指挥控制系统建设基本概念出发,探讨了陆战场空域指挥控制与空军空管、海军空管及民航空管之间的关系,分析了作战空间多维、战场态势多变、参战对象多元及指挥方式多样对于陆战场空域指挥控制所提出的需求,最后提出了陆战场空域指挥控制系统应具备的基本功能以及建设的基本途径。  相似文献   
60.
河南省农业水土资源空间匹配格局对产能的影响   总被引:1,自引:0,他引:1  
通过对河南省农业水资源与耕地资源的利用态势进行模拟分析,构建区域水土资源匹配测算模型,将匹配结果与产能分布情况做相关性分析,探讨河南省农业水土资源匹配格局对粮食产能的影响.结果表明,河南省耕地资源优势明显,水资源匮乏,水土资源匹配较差;水土资源匹配程度与耕地产能的空间分布呈现出明显的错位现象,这是由于产能高产区垦殖率高,灌溉条件好,过度利用水资源引起的.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号